1. Field of the Invention
The present invention relates to a novel bipolar membrane and a method for producing it. More particularly, the present invention provides a bipolar membrane comprising a cation-exchange membrane in which a cation-exchange group present at least at its adhered surface has been ion-exchanged with a heavy metal ion, which is adhered closely to an anion-exchange membrane at a peel strength of not less than 0.2 kg.multidot.f/25 mm, and having a water-splitting current efficiency of not less than 80% and a water-splitting membrane potential of not more than 2.0 V.
2. Description of the Prior Art
Bipolar membranes are widely used as diaphragms in the electrolysis of water (water splitting) or as separating membranes used in reclaiming acids and alkalis from aqueous solutions of salts that are products formed by neutralization between acids and alkalis, and ion-exchange membranes known in the art. They have a basic structure comprised of a lamination of a cation-exchange membrane and an anion-exchange membrane, and various methods for their production have been proposed.
The methods can be exemplified by a method in which a cation-exchange membrane and an anion-exchange membrane are laminated using a mixture of polyethyleneimine and epichlorohydrin and then bonded to each other by curing (Japanese Patent Publication No. 32-3962); a method in which a cation-exchange membrane is bonded to an anion-exchange membrane by using an adhesive having the properties of exchanging ions (Japanese Patent Publication No. 34-3961); a method in which a pasty material comprising vinyl pyridine and an epoxy compound is coated on the surface of a cation-exchange membrane, followed by exposure to radiations to obtain the product (Japanese Patent Publication No. 38-16633); a method in which a sulfonic acid polymeric electrolyte and an allylamine are adhered to the surface of an anion-exchange membrane, followed by exposure to-ionizing radiations (Japanese Patent Publication No. 51-4113); (Japanese Laid-Open Patent Application No. 53-37190); and a method in which a polyethylene film is impregnated with styrene and divinylbenzene followed by polymerization to give a sheet-like material, which was then nipped between frames made of stainless steel, where one side thereof is sulfonated, and thereafter, the sheet is detached and the remaining side is chloromethylated followed by treatment for amination (U.S. Pat. No. 3,562,139).
The bipolar membranes obtained by these methods, however, have the problem that a membrane potential much higher (e.g., 2.5 V to 3.0 V, or higher) than the theoretical water-splitting membrane potential (0.83 V) is applied when it is attempted to split off water, resulting in high-power consumption. They also can never have satisfactory current efficiency.
Electrochemica Acta, Vol. 31, No. 9, pp.1175-1176 (1986) reports a production method in which cation- and anion-exchange membranes whose surfaces have been coated with a solution comprising at least one kind of inorganic electrolyte selected from sodium tungstate, chromium nitrate, sodium metasilicate, and ruthenium trichloride are put together and then pressed to give a bipolar membrane having a low water-splitting membrane potential. This bipolar membrane, compared with the above numerous bipolar membranes, is characteristic of a low water-splitting membrane potential, which, however, is only at the initial stage of use. Thus, this membrane is disadvantageous in that the water-splitting membrane potential increases in a relatively short time, bubbles or blisters may occur at the interface between the cation-exchange membrane and anion-exchange membrane which is then of no use, and also, depending on conditions in its use, the bipolar membrane may separate into the cation-exchange membrane and the anion-exchange membrane which is then of no use. It also cannot satisfy current efficiency, and can not be said to be a bipolar membrane that can be of satisfactory use on an industrial scale.